For metering liquid fuels, piston pumps are customarily used because essentially it is only with piston pumps that fuels of pronouncedly different viscosities can be volumetrically metered. There are considerable practical problems with these piston pumps however, so that in general a disturbance-free operation is not ensured. In the first place, these metering pumps can only overcome low suction heads. Particularly in the case of fuels with a low boiling point, even at a suction head of a few centimeters, vapor bubbles occur in large numbers, whereby the amount delivered and also the hydraulic efficiency is substantially reduced. Depending upon the design and environmental conditions, the effect of vapor bubbles can be so great that the hydraulic efficiency will drop to zero.
Another drawback of these pumps, particularly in pumping fuels with a low boiling point, is the great dependency of the quantity that is pumped upon ambient temperature. These pumps always have to be mounted in the coldest possible place in the vehicle, and this often leads to difficulties and to faulty assembling. With ambient temperatures above 10.degree. C., the proportion of vapor bubbles increases, and the low suction power is further deteriorated. Similar difficulties also occur in the case of conduit lengths of several meters, which are usual in practice. Moreover, the filling time for dry conduits is considerable because the hydraulic efficiency of these pumps is very low if they have to drive gas. To obviate the difficulties that have been described, a second pump is often introduced upstream of the metering pump. This second pump is generally a diaphragm pump with its own electric drive. In this way increased conduit resistance is supposed to be compensated, excessive filling times in starting operation with lines that are still dry are to be avoided, and greater suction heads are obtained. However, it has been found that as before, the amount delivered to the metering pump is strongly dependent upon the pumping head. A change in the head by changing the liquid level in the storage tank produces unacceptable changes in the delivered quantities. The problems that arise when the ambient temperature is high are not solved by the upstream second pump either. The amount supplied by the diaphragm stage cannot be processed further in the heated metering stage. This means that with this solution also, the metering pump has to be mounted in a location that is as cool as possible.
The invention concerns the problem of producing an electromagnetic fuel delivery and metering pump in which the metered volume will remain constant even with long conduit lengths, up to 10 m for example, with suction heads up to 1 m for example, and ambient temperatures of 60.degree. for example. It is a further problem of the invention, to produce a pump of this type with the least possible number of individual parts.
The problem is solved according to the invention by a pump that presents a diaphragm pump that delivers in excess and a metering piston pump, whereof the working chambers can be interconnected when the diaphragm pump executes its delivery stroke and the metering pump executes its suction stroke. The piston of the piston metering pump is connected with an armature of an electromagent, and a common return spring is provided for the delivery stroke of the diaphragm pump and the suction pump of the piston metering pump.
An excess volume of fuel is sucked from the storage tank via the diaphragm pump. This excess can be adjusted as required as a function of the given diameter, over the diaphragm stroke. The physical properties of the fuel, the ambient temperature and the flow resistance of the pump circuit govern this adjustment. This excess is guaranteed in all operational states so that fluid is always presented to the metering stage. The diaphragm sucks the medium to be delivered via a suction valve and sends it via a pressure valve and a connecting conduit into a liquid store upstream of the metering stage. Both valves are necessary, to make the metering stage as independent as possible of resistance of the pump circuit, and to ensure capability for driving gas.
Suction through the pistion metering pump from the liquid store is from a lower region, whereas the return flow to the tank is from an upper region of the store. In this way it is ensured that any vapor bubbles that may be present in the storage tank will be separated out and cannot be sucked by the metering piston pump. Alternatively, the piston metering pump can also suck directly from the working chamber of the diaphragm pump via a suction valve. In this way the diaphragm of the diaphragm pump can be disposed directly on an extension of the piston of the metering piston pump. The end of the piston extension that penetrates into the working chamber of the diaphragm pump can be applied in a known way by the return spring against a seal in the inoperative position of the pump, whereby a tank cutoff is obtained, to prevent emptying of the tank in the inoperative state when the tank is high up in its disposition.
With the disposition of the diaphragm on an extension of the piston of the metering piston pump, the suction stroke of the diaphragm pump necessarily is executed together with the delivery stroke of the metering piston pump. In order, however, to allow an independent adjustment of the delivered volumes from the diaphragm pump and the metering piston pump, the diaphragm can be impinged upon by a special spring for execution of the suction stroke. If independently adjustable means are provided for limiting the piston movement in both directions, there can be adjustment on the one hand of the delivery stroke of the diaphragm pump effected by the common return spring, and on the other hand of the delivery stroke of the piston of the metering piston pump. The common return spring has an appropriate force to overcome the counter force of the spring that executes the suction stroke of the diaphragm pump. Accordingly, this latter spring can only act when voltage is applied to the magnetic coil and the piston of the metering piston pump moves.
When the piston of the metering piston pump moves with voltage applied to the magnetic coil, the medium present in the pump chamber will be thrust out via a pressure valve, to the consumer. After the switching off of the magnetic coil, the return spring pushes the piston back again into its starting position, and thereby the suction valve of the metering piston pump opens and the medium is sucked from the volume delivered by the diaphragm pump.
Because of the great delivery capacity of the diaphragm pump, the suction lines and possibly the storage chamber are rapidly filled with liquid.
Particularly in the case of fuels with low boiling point, there are always vapor bubbles in the suction line and in the valves. These bubbles are forced through the diaphragm into the liquid store and are separated here. The metering piston pump therefore always sucks liquid only. The diaphragm pump ensures not only a constant upstream pressure of the metering piston pump, but because of the liquid excess that can be selected, intrinsic heat and external heat is also carried off. Consequently, the tendency to bubble formation is substantially reduced even when the ambient temperatures are high. Since the diaphragm of the diaphragm pump is moved at least in one direction together with the piston of the metering piston pump, no special drive is necessary for the diaphragm pump. Hereby the simplification and cost reduction that are sought are obtained.
In the reflux conduit through which the fuel that was delivered in excess is carried back to the tank, according to a preferred embodiment, a check valve is provided and upstream of this check valve there is a surge tank. With such an arrangement, fluctuations in pressure on the suction side of the metering piston pump will be damped. These pressure fluctuations occur because of pulsed flows that are produced by the diaphragm and the piston. The check valve suppresses fluctuations in pressure that could occur in the reflux conduit. Such pressure fluctuations can allow the occurrence of different pressures because of a varying level in the storage tank, and without the check valve these differences in pressure would act directly on the suction side of metering piston pump. Instead of a check valve there could also be a choke, which has a similar effect.
These and further objects, features and advantages of the present invention will become more obvious from the following description when taken in connection with the accompanying drawings which show, for purposes of illustration only, several embodiments in accordance with the present invention.